There are various types of lasers, but in terms of excitation manners, there are mainly three types of lasers: a stimulated emission laser, a free-electron laser, and a chemical laser. Currently, lasers widely used in the industry are mainly stimulated emission lasers, which are classified into a solid-state laser, a gas laser, a liquid laser, a semiconductor laser, and the like according to difference of states of a working substance. A main characteristic of these lasers such that a laser wavelength is related to an excitation substance; and if the excitation substance is determined, the laser wavelength is determined accordingly.
A free-electron laser is a special type of new lasers. Its working substance is a directional free-electron beam which moves at a high speed in a magnetic field with a periodic space change. Tunable coherent electromagnetic radiation can be produced merely by changing a speed of the free-electron beam. In principle, coherent radiation spectrum can transit from an X-ray waveband to a microwave region, and a wavelength range of the electromagnetic radiation is adjustable. Therefore, the free-electron laser has a very promising prospect, and more implementation manners are needed, so that in a practical application, an appropriate selection can be made according to advantages and disadvantages of the various manners.